1. Ruby Containers, Iterators, and Blocks
CSE 413 Autumn 2008
Ruby Containers, Iterators, and Blocks

2. Containers in Ruby
Ruby has general, easy-to-use container classes, like most scripting languages
Two major kinds
Arrays: ordered by position
Hashes: collections of <key, value> pairs
Often known as associative arrays, maps, or dictionaries
Unordered

3. Ruby Arrays Instances of class Array
Create with an array literal, or Array.new
words = [ “how”, “now”, “brown”, “cow” ]
stuff = [ “thing”, 413, nil ]
seq = Array.new
Indexed with [ ] operator, 0-origin; negative indices count from right
words[0] stuff[2] words[-2]
seq[1] = “something”

4. Ruby Hashes Instances of class Hash
Create with an hash literal, or Hash.new
pets = { “spot” => “dog”, “puff” => “cat” }
tbl = Array.new
Indexed with [ ] operator
pets[“puff”] pets[“fido”]
pets[“cheeta”] = “monkey”
(Can use almost anything as key type; can use anything as element type)

5. Containers and Iterators
All containers respond to the message “each”, executing a block of code for each item in the container
words.each { puts “another word” }
words.each { | w | puts w }

6. Blocks
A block is a sequence of statements surrounded by { … } or do … end
Blocks must appear immediately following the method call that executes them, on the same line
Blocks may have 1 or more parameters at the beginning surrounded by | … |
Initialized by the method that runs the block

7. Blocks as Closures Blocks can access variables in surrounding scopes
all_words words.each { | w | all_words = all_words + w + “ ” }
These are almost, but not quite, first-class closures as in Scheme (some differences in scope rules)

8. More Block Uses
Besides iterating through containers, blocks are used in many other contexts
3.times { puts “hello” }
n = times { | k | n += k } puts “sum of 0 + … + 99 is ” + n
We’ll see more examples of blocks as well as how to write code that uses them later